The viscosity of synthetic rubber latex comprising polybutadiene at a given temperature and solids content is largely determined by the average particle size and the distribution of particle size of the polybutadiene. Generally, a large average particle size is desirable and results in a lower viscosity latex at a given solids content and temperature. Various techniques have been employed to increase the particle size of a rubber latex during emulsion polymerization. However, difficulties have been encountered with each of these techniques.
For example, Chittenden et al, Industrial and Engineering Chemistry, 40, 337 (1948), and Borders et al, Industrial and Engineering Chemistry, 40, 1473 (1948), disclose the use of high organic monomer to water ratios or low soap concentrations to promote agglomeration during emulsion polymerization reactions of butadiene. However, this technique results in one or more periods of instability during which coagulation may occur. Additionally, heat removal may be difficult because of high viscosity conditions and, if the reaction proceeds in an uncontrolled manner during the critical coalescent stage, it is often difficult to reproduce desirable reaction results.
It is also known to add colloidally active compounds to a rubber latex in order to increase the particle size. For example, the Howland et al U.S. Pat. No. 3,056,758 teaches the addition of polyvinylmethyl ether to a synthetic rubber latex in order to increase the particle size while the Schluter U.S. Pat. No. 3,330,795 discloses the addition of a polyalkylene oxide to a synthetic rubber dispersion to cause agglomeration. Attempts have also been made to add colloidally active compounds to the emulsion polymerization recipe. For example, Belgian Patent No. 817,505 discloses the addition of a polyalkylene glycol agglomerating agent while Araki, Polymer Journal, Vol. 19, No. 7, 863 (1987) discloses the addition of sodium alganate and magnesium sulfate to the emulsion reaction. However, these methods are disadvantageous in that oftentimes low monomer conversion rates are realized resulting in reactions that may require as long as 70 hours to reach completion.
The Gauslaa U.S. Pat. No. 3,318,831 discloses a method for the production of large particle latices in short reaction times using high-sheer techniques during polymerization. However, this method is disadvantageous in that it requires special reactors capable of high agitation rates.
Thus, a need still exists for improved methods for providing synthetic rubber latices having increased particle size.